We compared the regional myocardial distribution of the commonly used flow indicators; flow limited iodoantipyrine, partially flow limited rubidium chloride and 9uM microsphere. The analysis of such data, given the appropriate mathematical model, may give answers to a) the correction factors for flow measurements, b) distribution of permeability surface area product in the heart and c) flow heterogeneity in the beating heart, working or non-working. We will try and correlate regional blood flow to regional tissue contents of high energy phosphate compounds (obtained from myocardial biopsy) in the working heart. From this we shall attempt to delineate the determinants of supply and demand in heart muscle. BIBLIOGRAPHIC REFERENCES: Yipintsoi, T.: Single-Passage extraction and permeability estimation of sodium in normal dog lungs. Circ. Res. 39: No. 4 523-531, October, 1976. Bardfeld, P.A., Yipintsoi, T., Koerner, S.K., Crane, R., Hagstrome, J.W.C., and Veith, F.: 131I fibrinogen in the detection of pulmonary allograft rejection. Surgery 80: No. 5, 575-580, November, 1976, in press.